Rear projection apparatuses (hereinafter rear projector) where projected light from a light modulator, which forms an image by modulating light based on an image signal, is projected by a projection lens onto a screen from behind the screen are conventionally known.
A lens apparatus equipped with a moving lens unit is disclosed by Japanese Laid-Open Patent Publication No. H08-327872. This lens apparatus includes a lens barrel that has a plurality of cam grooves provided on an outer circumferential surface thereof in parallel with a central axis direction, a cam ring that is disposed in the lens barrel and includes a plurality of cam grooves provided so as to intersect the central axis of the lens barrel at an angle, a movable lens unit disposed inside the cam ring, a plurality of cam pins disposed on an outer circumferential surface of the movable lens unit, and a cam pin holding frame that is linked to the outer circumferential surface of the movable lens unit and supports the cam pins. This lens apparatus is constructed so that the cam pins and the cam pin holding frame are provided with through-holes that integrally pass through the cam pins and the cam pin support frame, and by screwing adjustment screws that contact the outer circumference of the movable lens unit into the through holes, the outer circumference of the movable lens unit is pushed from outside by the adjustment screws.
Rear projector magnify and project images formed by a light modulator (also referred to as a “light valve” or an “image forming apparatus”) such as a liquid crystal display apparatus or a DMD (digital mirror device) onto a screen from behind the screen using a lens unit (i.e., a projection lens). Rear projector can produce a large image in spite of having a slim form, and are subject to attention as next-generation image displaying apparatuses. Since the positional relationship between the light valve and the screen is fixed, the focus of the lens unit should preferably be fixed.
In a lens unit equipped with a plurality of lenses, even if a fixed focus-type unit, the individual lenses that compose the lens unit and the lens holder (or “lens holding unit”) that holds such lenses includes manufacturing tolerances. Accordingly, it is necessary to finely adjust the relative positions of the lenses on the optical axis. In addition to the differences between individual lens units, there are differences between individual rear projectors. Accordingly, there is demand for a design where the focus can be finely adjusted when incorporating a lens unit into a rear projector.
On the other hand, since the projection lens unit of a rear projector magnifies and projects an image formed by a light modulator, even a minute deviation in the focus may cause deterioration in the quality of the image formed on the screen. In particular, it is hoped that rear projectors will be able to replace displays where a light valve is directly viewed, such as cathode ray tubes, liquid crystal display apparatuses, and plasma displays. Accordingly, the images projected onto the screen of a rear projector are required to be as sharp as the images on a conventional display that is viewed directly. Also, after the lens unit has been installed in a rear projector with the focus and other conditions having been finely adjusted, it is preferable to prevent occurring deviation in the focus due to an external shock applied to the apparatus during shipping from the factory, during delivery to the user, or the like.